Technical Field
This application relates generally to circuits that may be used in battery chargers and other types of energy transfer systems, and more specifically to a bidirectional direct current/direct current (DC/DC) converter that may be used in a bidirectional battery charger for an electric vehicle, among other applications.
Background Information
Fast electric chargers with high efficiency are in high demand for charging battery powered devices, such as electric vehicles. For electric vehicles, typical alternating-current (AC) slow chargers include Level 1 chargers that take 120 volt (V) AC, and Level chargers that take 240V AC. Such slow chargers are typically limited to about 1.8 kilowatts (kW) in the case of Level 1 chargers, and 7.2 kW in the case of Level 2 chargers. Certain attempts have been made to produce AC fast chargers with high efficiency. By using components that have greater than 97% efficiency, the overall efficiency of some AC fast chargers may be higher than 94%, in contrast to the less than 91% of most Level 1 and Level 2 AC slow chargers.
One example of an AC fast charger that may provide these properties is shown in FIG. 1. In this design 100, the electrical grid 110 is coupled to a primary side alternating current/direct current (AD/DC) rectifier 120. The primary side AD/DC rectifier 120 provides a first direct current (DC) bus voltage VDC to a direct current/alternating current (DC/AC) inverter 130 of a mono-directional inductor-inductor-capacitor (LLC) resonant DC/DC converter 140. The DC/AC inverter 130 is coupled to a primary side of a transformer T that transfers power to a secondary side coupled to a secondary side AC/DC rectifier 150. The secondary side AC/DC rectifier 150 includes diodes D1, D2, D3, and D4 arranged in a bridge configuration. The output of the secondary side AC/DC rectifier 150 is a voltage Vb on a second DC bus that may be coupled to a battery (having resistance Rb) of an electric vehicle.
While such a fast charger design is useful, it has certain limitations. More and more applications require bidirectional power transfer, where, in addition to supplying DC power (e.g., to charger a battery), AC power can also flow back in the other direction to the electrical grid. For example, there is an increasing interest in vehicle-to-grid (V2G) systems where the batteries of grid-connected electric vehicles are used to supply power back as part of a demand response service. The above discussed AC fast charger design is incapable of transferring power back to the electrical grid due, at least in part, to the diodes D1, D2, D3, and D4 used in the secondary side AC/DC rectifier 150.
What is needed is a bidirectional DC/DC converter that may be used in, among other energy transfer systems, a bidirectional AC fast charger, and which may retain various desirable properties (e.g., high efficiency) of certain mono-directional DC/DC converter designs.